Before you start to check the masks

Standard naming convention for submitted masks

PIs have to use a standard naming
convention when they submit the masks. This is to avoid confusion as
to which mask name is associated with the ODF. The submitted masks
should have the following naming convention: G(N/S)YYYYSQPPP-XX_ODF.fits (N/S indicates North or
South). Here the YYYY is the year,
S the semester, PPP is the program number and XX is the mask number (e.g. GS2007BQ038-01_ODF.fits). If
the naming conventions are not the standard, then ask the PIs to
remove the masks from the Observing Tool (OT) and re-submit them with
the correct names. Note that the root names with the numbers (e.g.
GS2007BQ038-01) correspond to the names of the masks that are added to
the field “Custom Mask MDF’’
in the GMOS static component in the
OT Phase 2. An example of a set of masks submitted with the wrong
names is given in Figure 1.

Pre-imaging distribution for mask design checks

The pre-images required to check the masks are provided by the Gemini Staff. The processed images are uploaded using the OT file attachment facility.

Downloading the pre-images and the ODF files

PIs use the OT file attachment facility to upload the
ODF fits files. Download the ODF fits files and the pre-images into your local
computer using the Fetch button in the "FileAttachment’’
window (see the example below). To download the fits files, the OT will ask
for your NGO password. Verify that the PIs supplied the names of the pre-images
used to create the ODF files. In the example below (Fig. 1), the name of the
pre-imaging is inserted in the Description field
in the File Attachment window. However, the
description could be inserted also in a Note inside OT Phase 2 program.

Figure 1: File attachment window with the details about the submitted
files (could be ODF fits files or finding chart). Note that in this example,
the mask names are not the standard name. The Description
Field shows the name of the pre-images used to generate the masks. The
description could be also presented as a Note in the OT Phase 2 program.

Starting the Gemini Mask Making Software (gmmps)

To check the mask design, you have to install the gmmps program. Here we will assume that gmmps is installed in your computer and is working
properly. Start gmmps. Go to File --> Open
and load the distributed pre-imaging. Select the HDU 2
in the FITS HDU window (see Figure 2). You can
adjust the intensity levels by using View --> Cut Levels. A 98% level should work fine in most of the cases (you
can see in detail the bright and the faint objects).

Figure 2: Portion of the gmmps main window with the FITS HDUs (1) window.

For ODF fits file. Go to GMOS-MMS --> Convert ODF fits to catalog --> Convert ODF fits to Cat. File….…, and load the ODF fits file. A new window will be
displayed (see Figure 3). This window contains information about the targets,
i.e. ID, RA, DEC, XCCD, YCCD, slitpos_x, slitpos_y slitsize_x, slitsize_y,
slittilt, MAG, priority, slittype, etc... The figure below shows the main
parameters listed in the ODF fits file. Objects with priority "0’’ are the alignment stars (always in the first
rows). The size of thealignment boxes
are 2 x 2 arcsec and is constant. The slit width is given by the column
"slitsize_x’’ (in the example is 0.75 arcsec) and the slit length is given by
the column "slitsize_y’’ (arcsecs).

Figure 3: Window containing all the column of the designed
mask (ODF). The column slitsize_x contains the slit
width (in this example 0.75”). The alignment stars have slitsize_x, slitsize_y 2x2 arcsec and have priority 0.

Click Plot Slits to draw the slits over the image.
Display the location of GMOS CCD gaps by clicking on Plot GMOS Gaps
(see Fig. 3). The figure below (Fig. 4) shows the object
slits (white and yellow), the alignment stars
(cyan) and the GMOS gaps (blue) plotted over a
GMOS image. The red rectangle indicates the mask
area. No objects will be cut outside this area.

Figure 4: Object slits (white and yellow) with the alignment stars (cyan) , CCD gaps (blue) and the mask area plotted over the pre-imaging. This is the visualization used to check the masks.

Object Slits (science targets)

Check that the size of the red box and the slits are at least
the size of the pre-imaging. If the size is double or half,
then there is a problem with the pixel
scale (the PIXSCALE keyword
inside the ODF fits table is wrong
and does not match the pixel scale in the pre-imaging). An example is
given below (see Fig. 5). In this case, the mask was designed using a
pixel scale of 0.0727”, but the pre-imaging was observed using a
2x2 binning (pixel scale of 0.1457” per pixel). You can see that
the slits and the red box are double the size of the pre-imaging. Send
the mask back to the PI and ask to use the correct pixel scale
(probably, the PI did not use the mostool
package to generate the Object Table).

Figure 5: An example of a ODF fits file with the wrong pixel scale. Here the pixel scale of
the ODF fits file is double the pixel scale of the pre-imaging.

Verify that there are no object slits located outside the red box area. Object slits outside this area are not included for cutting.

Check if the targets are inside the slits. In
particular, pay attention to the tilted slits. In the GMMPs
version 0.22 the tilted slits are not visualized. In the current version of
the software this problem has been corrected. Check the
column slittilt for objects with tilted slits. Positive angles indicate that the slits are tilted counterclockwise, while negative angles indicate that the slits are tilted clockwise. In general, the objects lie in the center,
however some people add tilted slits to get two or more objects in the same
slit.

For tilted slits, slit tilt must be within 45 < tilt < -45 deg.

Check the slit width. The GMOSs have a set oflongslit with the following width (normal and Nod & shuffle): 0.25”,
0.5”, 0.75”, 1”, 1.5”, 2” and 5”. If the width of the slits are different (e.g.
0.8”), then you have to tell the PI that any required calibration (flux
standard, radial velocity standard, etc) will be observed with a different slit
width, close to the width used in the mask (for 0.8”, will be 0.75” longslit).

Alignment objects

There should be at least 2 good
alignment objects for masks designed from pre-imaging, and 3 good alignment objects
for masks designed from object catalogs. The best masks are designed with 3 or 4 alignment objects. Some PIs have
concerns about the mask alignment and add more than 6 alignment objects in the
mask. This is not necessary. If this is the case, then
contact the PI and warn them about this. A high number of alignment objects
will increase the overheads at the telescope.

Alignment objects should be well distributed ie. on opposite corners of the mask or on different chips.

Check that the alignment objects do not fall into the gaps (or
near the gaps). Example of alignment stars inside the gap and too close
to the gaps are shown below (Fig. 6). A good alignment object should be at
least 2” away from the border of the gaps. In this case, the PIs have to select
another alignment star and re-do the masks.

Figure 6: The figure shows two examples of alignment objects
located near or over the GMOS gap region. Left: alignment star too close to the
gap. Right: alignment star in the gap.

Check for saturated
alignment objects. The software used to align the mask is not working properly
if one or more alignment starts are saturated. A
saturated star will have the wrong X,Y
coordinates (i.e. the centroid of the object is not good). This will introduce
an offset in the alignment object position and will affect the accuracy of the
on-sky alignment (and for the objects in the slits).

Verify that the alignment objects are bright enough for a 30sec exposure (common exposure
time used for acquisition). A good rule of thumb is that alignment stars should have magnitudes between ~14-20.

Verify that the magnitudes of
the alignment objects are similar. Big differences in magnitude can
compromise the mask alignment. Ideally, brightnesses will be within about 2 magnitudes. If the magnitudes are not provided, then check
that the apparent brightnesses in the pre-imaging are similar.

Check that the alignment objects are not galaxies. In principle, we can use galaxies to center the mask, but the
solution would not be as good as if you use stars.
An example is given in Figure 7. If this is the case, then ask the PIs to
replace the objects by stars or, if there are few other alignment stars,
eliminate them from the masks.

Figure 7: Example of galaxies used as alignment objects. In
this example, the PI designed a mask with 6 alignment objects. The other four
are stars. The PI has removed these galaxies from the mask and leaves only the 4
alignment stars.

Nod & Shuffle masks

N&S MOS mask design requires special
attention. There are two shuffling modes that you one choose from when
observing with Nod & Shuffle: 1) Band-shuffling; 2) Micro-shuffling. Here
we describe the two modes and show how to check the N&S MOS mask design.

Band-shuffling

The simplest possible layout is band shuffling with a single science band.In this case the middle third of the detector is the only science region, and the
top and bottom thirds of the detector are used for storage exclusively.
This band layout allows one to place a very high density of slits of variable
length in a centralized region; however, one loses 67% of the field of view for
slit placement.

The slits can be any length. Verify that all the slits are in the centerof the image and that the
three bands have identical size. Figure 8 shows an example of single
science band.

Figure 8: Example of band shufflingwith
asingle science band.All
slits are located in the middle third of the detector. The top and bottom
thirds of the detector are used for storage exclusively.

The next example has two
science bands and three storage bands
(see Fig. 9 below). Because storage regions between science regions can be
shared, one starts to win back field of view as the number of bands is
increased. Here we are using 40% of the GMOS detector for science (available
for slit placement) and 60% for charge storage. Once more each of the science
bands can contain a high density and number of slits of variable length. Each
band must have the same height and there must be two storage regions
immediately adjacent to each science region; however, they do not need to be
regularly spaced or fill the entire detector as shown here.

The slits can be any length. Verify
that all the slits are contained within one of the two science bands. Also, verify that the two science bands contain the same number of rows and that the three storage bands are at least the same size as the science bands. (See the example in Fig. 9 for a two band
shuffling).

Figure 9:Example of band shuffling with
a two science bands. All slits are located in
the two band occupying 40% of the detector. The remaining 60% of the detector
is used for storage exclusively.

Micro-shuffling.

The limiting case is of many bands where each science
region contains exactly one slit and, therefore, each
slit has the same length. This special case is named micro shuffling. As the number of micro-shuffled bands
increases and the size of the slits decreases one can use nearly 50% of the
GMOS field of view for slit placement. When using micro shuffling:

Check that all the slits have
the same length

Check that the minimum space
between slits has at least the same length as
the slit.

Check that the slit length
and the offset in q-direction in the OT are consistent. In the example given in
Fig. 10, the slit length is 3”. A nod offset of +-0.75” would keep the object in the slit for both nod positions.

Figure 10: Nod & Shuffle window inside the GMOS static
component. In this case, a nod distance of +- 0.75” is good enough for a slit
length of 3”.

An example of micro-shuffling mask is given in Figure
11. Note that the spaces between the slits are bigger than the slit lengths.

Figure
11: Example
of a micro-shuffling mask (slit length of 2”).

Nod offsets

Check that the nod offsets are consistent with the defined slit length. For example, a nod offset of (0, 10) and (0,-10) would be completely inconsistent with a slit length of 3 arcsec. However, nod offsets of [0,0] and [0,35] (in which case telescope nods between the science target and a blank sky position) or nod offsets of [0,+0.75] and [0,-0.75] (in which case the telescope nods the science up and down within the slit) would both be compatible with 3 arsec long slits.

Shuffle distance

Check that the shuffle distance, SHUFSIZE, in the mask file header, and the shuffle distance
provided in the Nod & Shuffle window inside the GMOS
component in the OT are the same. For the mask, you can’t use the “gmmps”. To
check this value in the mask, you can use the “tprint” task in table.ttools
inside IRAF. The command is the following:

gm> tprint
maskname-01_ODF.fits prparam+ prdata-

With this command you will list the header
parameters inside the masks. An example is given below.

Figure 12: Output from tprint IRAF command. The shuffle
distance is given in the field SHUFSIZE. In this example the shuffle distance
is 42 and should be the same that the value inserted in the field Offset (detector rows) in the Nod & Shuffle window
inside the GMOS static component.

Post-mask checking

If the masks have problems, then the NGOs will contact the PIs
and explain the nature of the problem. If the masks show any of the problems
explained above, the PIs have to re-design their masks and re-submit them again
using the OT file attachment facility. To do that, the PIs have to remove the previous submitted
masks and upload the new ODF fits files following the correct naming convention.

If the mask passed the quality control, the NGOs have to check
the "NGO check?" box in the "File Attachment" window in the OT (see Fig. 1). An
automatic e-mail alert is sent to the g(n/s)mascheck@gemini.edu
and the g(n/s)qc@gemini.edu exploders, and to
the Gemini Contact Scientists and NGO contacts.
The usual mask checker at Gemini will then double-check the mask design
and will iterate with the NGO, if it is necessary. The mask
checker at Gemini will run the program odf2mdf
to generate all necessary files for mask cutting (including the Mask Definition
File - MDF). Finally, the Gemini mask checker ingests all new masks into our
internal database and then the masks are cut.

All Phase II MOS sequences are put On
Hold following the normal PhaseII --> For Review --> For Activation
process at the beginning of the semester. After Gemini staff complete the mask checks and send an email notifying the PI and NGO that the masks are ready to be cut, the NGO will update the MOS observation status from "On Hold" to "For Activation", or back to "Phase 2" if the PI needs to further edit the observation.

If the mask was made from a catalog, NGOs or Contact Scientists should first check that the RA, Dec, and position angle for the relevant observation in the OT are identical to the values used to make the mask before setting the observation to "For Activation" or "Ready".
The RA and Dec used for designing the mask can be extracted from the MDF file or pseudo image (eg by using "thselect {maskfile.fits} RA_IMAG,DEC_IMAG" or "hselect {psuedo_image} RA,DEC in pyraf). Check the PA by comparing the pseudo image orientation (after inverting y and rotating to the PA listed in the OT) with eg. a DSS image in the OT position editor.

Checking the Masks

You should not expect to find any
problems with the mask design if the PI followed the mask design instructions. However, if you see any of the problems listed
below, immediately contact the PI and request they design
a new mask(s).